There are many popular file types for digital video files. AVI, MPEG and MOV are commonly seen video file extensions. While MPEG and MOV are more compressed--and therefore smaller--than AVI files, all of these file types create fairly sizable files, making them less than ideal for portable media devices,MP4 is a popular video file format with a small size, making it perfect for most potable players,such as psp ,ipod,so we need a program - mac mp4 converter